Shell problems for RES

I adopted 2 RES roughly more than 1 years old and recently i started to see some white patches on their shells. I do have a basking area and put the tank under direct sunlight, not sure if it is shedding or fungus infection. Need advice!!! What should i do and how to treat them?

Attached photos are the observations of my RESs

Attached Thumbnails

What I can tell from your pics is that it's not shedding...it looks like bacterial infections. My slider had a similar symptom a few years ago and a vet told me to treat her shell with iodine solution daily. The vet also told me that their commensal bacteria can propagate inside their shells very rapidly and form patches when turtles have immune system imbalance.
In my slider's case, bacterial patches propagated rapidly during the summer of 2009, but they are mostly gone now. I did not use iodine solution, but the patches got smaller and smaller each time she shed.